1. Field of the Invention
The present invention relates to a three-lens-group zoom lens system for a compact camera, and in particular, relates to increasing the zoom ratio of such a zoom lens system.
2. Description of the Related Art
In a zoom lens system for a compact camera, a long back focal distance is not necessary, unlike a zoom lens system of a single lens reflex (SLR) camera which requires a space to provide a mirror behind the lens system. Therefore in a compact camera, a telephoto-type lens system, including a positive lens group and a negative lens group, in this order from the object, is generally used. On the other hand, in a single lens reflex camera, a retrofocus-type lens system, including a negative lens group and a positive lens group, in this order from the object, is used.
In a compact camera, the zoom ratio of the telephoto-type zoom lens system is conventionally at most 2 to 3 in the case of the three-lens-group arrangement. Moreover, a three-lens-group zoom lens system having a zoom ratio of 4.0 or more has not been known.
It is an object of the present invention to provide a three-lens-group zoom lens system for a compact camera, which can attain a high zoom ratio of 4.0 or more.
In order to achieve the above-mentioned object, there is provided a zoom lens system including a negative first lens group, a positive second lens group, and a negative third lens group, in this order from the object. Zooming is performed by moving the first through third lens groups in the optical axis direction. The negative first lens group includes positive-and-negative cemented lens elements in which the most object-side surface is a concave surface. The zoom lens system satisfies the following conditions:
xe2x88x921 less than r1/fW less than 0xe2x80x83xe2x80x83(1)
1.4 less than f1G/f1N less than 1.8xe2x80x83xe2x80x83(2)
wherein
r1 designates the radius of curvature of the most object-side surface of the negative first lens group;
fW designates the focal length of the entire zoom lens system at the short focal length extremity;
f1G designates the focal length of the negative first lens group; and
f1N designates the focal length of the negative lens element of the cemented lens elements of the negative first lens group.
In the cemented lens elements, the order of the positive lens element and the negative lens element is not specifically required. Accordingly, the cemented lens elements can be constituted by the negative lens element and the positive lens element in this order from the object.
The zoom lens system according to the present invention preferably satisfies the following condition:
8 less than xcexdnxe2x88x92vpxe2x80x83xe2x80x83(3)
wherein
xcexdn designates the Abbe number of the negative lens element of the cemented lens elements of the negative first lens group; and
xcexdp designates the Abbe number of the positive lens element of the cemented lens elements of the negative first lens group.
The zoom lens system according to the present invention can satisfy the following condition:
xe2x88x925 less than fT/f1G less than xe2x88x923.5xe2x80x83xe2x80x83(4)
wherein
fT designates the focal length of the entire zoom lens system at the long focal length extremity.
The zoom lens system according to the present invention preferably satisfies the following condition:
0.03 less than (d12Wxe2x88x92d12T)/fW less than 0.10xe2x80x83xe2x80x83(5)
wherein
d12W designates the distance between the first lens group and the second lens group at the short focal length extremity; and
d12T designates the distance between the first lens group and the second lens group at the long focal length extremity.
The zoom lens system according to the present invention can satisfy the following condition:
0.6 less than y/fW less than 0.9xe2x80x83xe2x80x83(6)
wherein
y designates the diagonal image heighton a film surface.
The zoom lens system according to the present invention preferably satisfies the following condition:
3.5 less than fT/fWxe2x80x83xe2x80x83(7)
The zoom lens system according to the present invention can attain a zoom ratio of 4 or more. On the other hand, the zoom lens system of the present invention can also be applied to a zoom lens system having a zoom ratio of about 3.5.
In the zoom lens system of the present invention, it is preferable that a lens element having at least one aspherical surface be provided in the positive second lens group, and the following condition be satisfies:
xe2x88x9230 less than xcex94IASP less than xe2x88x9210xe2x80x83xe2x80x83(8)
wherein
xcex94IASP designates the amount of change of the spherical aberration coefficient due to the aspherical surface under the condition that the focal length of the entire zoom lens system at the short focal length extremity is converted to 1.0.
Furthermore, in the zoom lens system of the present invention, it is preferable that a lens element having at least one aspherical surface be provided in the negative third lens group, and the following condition be satisfied:
0 less than xcex94VASP less than 0.4xe2x80x83xe2x80x83(9) 
wherein
xcex94VASP designates the amount of change of the distortion coefficient due to the aspherical surface under the condition that the focal length of the entire zoom lens system at the short focal length extremity is converted (normalized) to 1.0.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2001-015741 (filed on Jan. 24, 2001) which is expressly incorporated herein by reference in its entirety.
Copending and commonly assigned U.S. Patent Application to xe2x80x9cA Zoom Lens Systemxe2x80x9d, listing as an inventor Takashi ENOMOTO, filed concurrently with the present application, having Attorney docket No. P20342; and Japanese Patent Application No. 2000-047619 (filed on Feb. 24, 2000), from which the application designated by Attorney docket No. P20342 claims priority, are expressly incorporated herein by reference in their entireties.